Because I believe your taking away all the happiness that person could gain in his life, taking a life can only be acceptable if you are taking the life of a creature who has greater suffering in comparison to it's happiness, however much ignorance can occur when making the decision because the judgement of another person's future happiness/suffering is very hard, one must take absolute full consideration of a person's suffering/happiness, therefore making this decision and it being rightful should be greatly awarded because it evolves a great amount of consideration.